1. Field of the Invention
The present invention relates to an elliptically polarizing plate and a liquid crystal display device that uses an elliptically polarizing plate.
2. Description of the Related Art
Polarizing plates have conventionally been used in liquid crystal display devices, and a case that can be cited as an example of this type of polarizing plate is described in "Polymer Film Technology" by Okada Toyokazu in Third Address of the Liquid Crystal Young Researchers Conference on page 106 of "Year 2000 Liquid Crystal Research." Explanation is next presented regarding a polarizing plate of the prior art based on this example. FIG. 1 is a schematic view illustrating the effect of a linearly polarizing plate of the prior art.
Polarizing plates that have been put to practical use for current liquid crystal display devices are components in which a dichroic substance is adsorption oriented to drawn-oriented polyvinyl alcohol. In other words, macromolecular chain 17 of a polyvinyl alcohol is uniaxially oriented, and dichroic substance 1 is oriented so as to follow this chain and adsorbed. The dichroic substance generally has a spheroidal absorption coefficient. As a result, polarized light that vibrates in the orientation of the major axis of the spheroid exhibits a large absorption coefficient and polarized light that vibrates in the orientation of the minor axis of the spheroid exhibits a small absorption coefficient.
Incident light 16 to a polarizing plate is generally in a non-polarized state. In cases in which this light is irradiated onto a dichroic substance arrayed as shown in FIG. 1, polarized light of the major axis orientation (y orientation) of the dichroic substance is strongly absorbed, and polarized light of the minor axis orientation (x orientation) is weakly absorbed. As a result, light in a substantially linearly polarized state can be obtained as emitted polarized light 18. This minor axis orientation (x orientation) is referred to as the transmission axis of the polarizing plate. The foregoing explanation gives the principles for emission of linearly polarized light by a polarizing plate employing a dichroic pigment.
A liquid crystal display device generally includes one pair of sheet polarizers. Particularly in liquid crystal display devices having a high contrast ratio, the transmission axes of two sheet polarizers are arranged orthogonally. To explain in more detail, the contrast ratio is defined as (the transmittance of a light state) divided by (transmittance of dark state). As a result, a dark state of low transmittance must be realized for the entire range of visible light in order to achieve a high contrast ratio. This state can be realized when the liquid crystal is perpendicular to the substrate surface between sheet polarizers having orthogonal transmission axes.
As an actual example, it is known that a homeotropic liquid crystal, in which a perpendicularly oriented liquid crystal is enclosed between polarizing plates having orthogonal transmission axes, has a high contrast ratio. It is further known that a case in which twisted nematic liquid crystal is enclosed between polarizing plates having orthogonal transmission axes also has a high contrast ratio. In these cases, a dark state can be obtained when voltage is impressed to the twisted nematic liquid crystal to bring about perpendicular orientation.
As described hereinabove, the high contrast ratio of the liquid crystal display devices results from polarizing plates having orthogonal transmission axes.
Recent years have seen the development of methods that provide remarkable improvement in the visual angle dependency of liquid crystal display devices. However, the visual angle dependency of the polarizing plates imposes an upper limit on such widening of the angle of field. This point is discussed in further detail hereinbelow.
The visual angle dependency of a single linearly polarizing plate of the prior art is first explained with reference to FIG. 2. A linearly polarizing plate of the prior art has a spheroid absorption coefficient as shown in FIG. 2. When viewed from the front [from the z axis], there is a difference between the absorption coefficient .alpha.e corresponding to the elliptic major axis, which is the x-axis azimuth, and the absorption coefficient .alpha.o corresponding to the elliptic minor axis, which is the y-axis; and this difference brings about the dichroic ratio. No change occurs in the two absorption coefficients .alpha.e and .alpha.o if the direction of view is inclined from the front to the A bearing as shown in FIG. 2, but absorption coefficient .alpha.o appears to decrease if the direction of view inclines to the B bearing of FIG. 2, and the dichroic ratio therefore appears to fall at the B bearing. For these reasons, visual angle dependency is great at particular bearing angles in a linearly polarizing plate of the prior art.
The visual angle dependency for a case in which the transmission axes of these linearly polarizing plates are arranged orthogonally is next considered. Transmittance rises in a case in which this pair of polarizing plates is viewed from an angle. This is because the transmission axes of the polarizing plates appear to diverge from 90 degrees when viewed from an angle even though the axes are orthogonal when viewed from the front. As a result, high transmittance is exhibited when viewed from an angle even though low transmittance is exhibited from the front. Accordingly, despite some improvement in the visual angle dependency of the liquid crystal display medium, the visual angle dependency of polarizing plates having orthogonal transmission axes still remains. As described hereinabove, the visual angle dependency of a polarizing plate determines the upper limit of the angle of field characteristic of a liquid crystal display device.